It has been found that passive immunization with relatively small amounts of the globulin fractions obtained from sera of mice either 3 or 14 days after a single injection of polysaccharides from Serratia marcescens afforded mice bearing sarcoma 37 pronounced protection against the lethal action of the homologous polysaccharide but not against that of the polysaccharides from a different strain of the organism.

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This research was supported in part by an American Cancer Society grant recommended by the Committee on Growth of the National Research Council, and by a grant from the National Institute of Health.

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